Accretionary Wedge
The significance of the Cascadia Subduction is that the Juan de Fuca plate is submerged under the coastal waters of the Pacific Ocean along the western edge of the North American Plate. A subduction is a condition where one plate pressing against another plate distorts and either climbs over the other plate, forming a mountain range, or dives under the opposing plate. The Juan de Fuca plate at the Cascadia subduction dives under the North American Plate creating a Accretionary Wedge.
A subduction causes extreme pressures both pushing against but also bending a flexing against the larger plate. When the fault line between the two slip to release some pressure it creates significant to catastrophic seismic events.
The USGS says that an Accretionary Wedge is made up of "Sediments, the top layer of material on a tectonic plate, that accumulate and deform where oceanic and continental plates collide. These sediments are scraped off the top of the down going oceanic crustal plate and are appended to the edge of the continental plate." See the Earthquake glossary
The fact that the subducting plate consists of significant amounts of water saturated material (dirt, silt, sand, etc.) it is taking that under the earth's crust with the plate. When that "wet earthen material" comes into the underlying magma chamber, it produces steam. The steam builds pressure underground like a tea kettle getting ready to whistle. In this case the whistle would be a volcanic or super-heated steam geyser explosion.
Any significant "kick" or slippage of the folded crust along could result in 9-10 magnitude earthquakes and potentially several hundred foot high tsunamis. The devastation would be biblical along most coastal areas today. The majority of the earth's populations live along the coasts.
Earth – Seismic / Techtonic Articles
Earthquake Intensity Scale
